This amended PPG application is designed to provide mechanistic insight into the mechanisms involved in the initiation and propagation of abdominal aortic aneurysms (AAAs). Projects will use a model of AAA in which angiotensin II will be infused into hyperlipidemic mice. This was described originally by faculty who are Project Leaders in this application. This model has now been adopted widely in both academic and pharmaceutical research programs. Many characteristics of the model reproduce features of the human disease, including the strong gender specific prevalence in males, fragmentation of medial extracellular matrix and presence of inflammatory cells. Studies on the cellular sequence of events in the development of AAAs demonstrate the involvement of inflammatory cells at all stages of development. The unifying hypothesis of this amended program project application is that angiotensin II promotes aneurysm formation by initiating regional-specific matrix degradation in the media of the abdominal aorta, leading to recruitment of macrophages, medial dissection, and subsequent vascular remodeling. Four projects are proposed that mechanistically define the sequence of events outlined in this unifying hypothesis. To facilitate research among Projects, 4 core facilities are proposed. This brings together a group of faculty who have interacted for several years. The synergy of their interactions is demonstrated by the multiple joint publications and co-investigator status on grants. These projects and cores have been designed to provide mechanistic insight into the development of AAAs in the following areas: Project 1 (Lisa A. Cassis) will study the role of male androgen in promoting regional-specificity of AAA formation through regulation of the angiotensin type 1A receptor. Project 2 (Alan Daugherty) will determine the role of smooth muscle cell specific AT1a receptors in the mechanisms of AAA initiation. Project 3 (Fred C. de Beer) will determine the role of serum amyloid A and its interaction with selected matrix metalloproteinases. Project 4 (Nancy R. Webb) will determine the role of specific isoenzymes of secretory phospholipase A2s in promoting inflammation at the site of macrophage recruitment during the evolving AAA. These projects will be supported by Cores of: A. Administration. B. Human studies. C. Mouse maintenance. D. Quantitative pathology.